According to the International Energy Agency, the current global energy supply is overwhelmingly originated from fossil fuel combustion, and only a small portion of our world-wide energy needs are supplied by renewable resources such as solar, wind, wave, geothermal and hydroelectric power sources. International Energy Agency, World Energy Outlook, OECD/IEA, Paris, 2011. Osmotic power systems generating salinity gradient energy are considered to be a promising alternative renewable energy source with an estimated power potential of approximately 2 TWh/year globally. Logan, B. E.; Elimelech, M., Membrane-based processes for sustainable power generation using water. Nature 2012, 488 (7411), 313-319; Ramon, G. Z.; Feinberg, B. I; Hoek, E M., Membrane-based production of salinity-gradient power, Energy & Environmental Science 2011, 4, (11), 4423-4434.
Osmotic power, or salinity gradient energy, is an energy that can released and harvested by mixing two solutions with different concentrations in a power cycle or system. In 1954, R. E. Pattie first suggested the presence of an untapped osmotic pressure source of power in geographic locations where a freshwater river mixes with sea water. Pattle, R. E., “Production of electric power by mixing fresh and salt water in the hydroelectric pile,” Nature, 174 (4431), 2 Oct. 1954. In the 1970s, Sidney Loeb outlined several practical methods for exploiting osmotic power using pressure retarded osmosis (PRO) and reverse electrodialysis (RED) systems. U.S. Pat. No. 3,906,250 to Loeb, issued Sep. 16, 1975 (PRO) and U.S. Pat. No. 4,171,409 to Loeb, issued Oct. 16, 1979 (RED).
Currently, pressure retarded osmosis (PRO), reverse electrodialysis (RED), and capacitive mixing (CM) have been investigated as the three major methods used to harvest salinity gradient energy; and, among these three methods, PRO is the most widely investigated salinity gradient energy technology. Lin, S.; Straub, A. P.; Elimelech, M., Thermodynamic limits of extractable energy by pressure retarded osmosis. Energy & Environmental Science 2014, 7, (8), 2706. Most of the research into pressure retarded osmosis (PRO) power systems has focused on the use of seawater as a draw solution and fresh river water as a feed solution, which requires the PRO power plant to be built in a geographic location near the interface of a freshwater river and the sea, or near another type of hypersaline water source, such as the Dead Sea or Great Salt Lake.
The problems currently associated with known osmotic power systems include the following: (1) limited geographic locations where there is an abundance of saline or hypersaline solutions and fresh river water used for feed and draw solution sources; (2) limited osmotic pressures that exist near sea level (seawater's osmotic pressure is approximately 27 bar, which is not sufficient to make PRO energetically competent); (3) membrane permeability problems limit the power system's economic feasibility (existing membranes are permeable for the sizes of Na+ and Cl− in the solute, which limits improvements without sacrificing selectivity); (4) environmental harm to certain animal or plant species resulting from the discharge of the brackish water from existing types of osmotic power systems, which can cause salinity fluctuations to sensitive ecosystems; and (5) net energy reductions resulting from energy required for operation of open systems, including energy consumed during intake, discharge, and pretreatment of feed and draw solutions.
The world's first PRO pilot plant was built by Statkraft, and this pilot power plant used the osmotic gradient between the sea and a nearby freshwater river. Akst, D., Wall Street Journal, Aug. 29, 2014 “A New Kind of Power From Salt Water;” Moskwa, W., Reuters, Nov. 24, 2009, “Norway opens world's first osmotic power plant.” The Statkraft pilot plant is reported to have generated a gross power output of between 2-4 kW. Plans for further development and construction of additional osmotic power plants has been suspended by Statkraft based on the problems experienced with the pilot plant operations.
Enhancing the draw solution's osmotic pressure can be achieved by choosing brines from RO plant or hypersaline lakes (e.g., Dead Sea, Great Salt Lakes), but this solution does not resolve all the problems mentioned above, although such modifications can enhance the power density achieved by prior art systems to a small degree. Helfer, F.; Lemckert, C.; Anissimov, Y. G., Osmotic power with Pressure Retarded Osmosis: Theory, Performance and Trends—A Review,” Journal of Membrane Science 2014, 453, 337-358. While other scholars have attempted to modify feed and draw compounds used in existing power systems, these modified systems do not resolve all of the above problems, which persist and remain unresolved prior to the conception and development of the present invention. McGinnis, R. L.; McCutcheon, J. R.; Elimelech, M., “A novel ammonia-carbon dioxide osmotic heat engine for power generation,” Journal of Membrane Science 2007, 305, (1-2), 13-19; 7 Al-Mayahi, A.; Sharif A. Osmotic Energy. 2006, US 2006/0225420.